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See also:HEARING (formed from the verb " to hear," O. Eng. hyran, See also:heron, &c., a See also:common See also:Teutonic verb; cf. Ger. Koren, Dutch hooren, &c.; the O. See also:Tent. See also:form is seen in Goth. hausjan; the initial h makes any connexion with " See also:ear," See also:Lat. audire, or Gr. &Kobew very doub tful), in See also:physiology, the See also:function of the See also:ear (q.v.), and the,See also:general See also:term for the sense or See also:special sensation, the cause of which is an excitation of the auditory nerves by the vibrations of sonorous bodies . The See also:anatomy of the ear is described in the See also:separate See also:article on that See also:organ . A description of sonorous vibrations is given in the article See also:SOUND; here we shall consider the transmission of such vibrations from the See also:external ear to the auditory See also:nerve, and the physiological characters of auditory sensation . 1 . Transmission in External Ear.—The external ear consists of the pinna, or See also:auricle, and the external auditory meatus, or See also:canal, at the bottom of which we find the membrana tympani, or See also:drum See also:head . In many animals the auricle is See also:trumpet-shaped, and, being freely movable by muscles, serves to collect sonorous waves coming from various directions . The auricle of the human ear presents many irregularities of See also:surface . If these irregularities are abolished by filling them up with a soft' material such as See also:wax or oil, leaving the entrance to the canal See also:free, experiment shows that the intensity of sounds is weakened, and that there is more difficulty in judging of their direction . When waves of sound strike the auricle, they are partly reflected outwards, while the See also:remainder, impinging at various angles, undergo a number of reflections so as to be directed into the auditory canal . Vibrations are transmitted along the auditory canal, partly by the See also:air it contains and partly by its walls, to the membrana tympani . The See also:absence of the auricle, as the result of See also:accident or injury, does not cause diminution of See also:hearing . In the auditory canal waves of sound are reflected from See also:side to side until they reach the membrana tympani . From the obliquity in position and See also:peculiar curvature of this membrane, most of the waves strike it nearly perpendicularly, and in the most advantageous direction . 2 . Transmission in See also:Middle Ear.—The middle ear is a small cavity, the walls of which are rigid with the exception of the portions consisting of the membrana tympani, and the membrane of the See also:round window and of the apparatus filling the See also:oval window . This cavity communicates with the pharynx by the Eustachian See also:tube, which forms an air-tube between the pharynx and the tympanum for the purpose of regulating pressure on the membrana tympani . During See also:rest the tube is open, but it is closed during the See also:act of deglutition . As this See also:action is frequently taking See also:place, not only when See also:food or drink is introduced, but when saliva is swallowed, it is evident that the pressure of the air in the tympanum will be kept in a See also:state of See also:equilibrium with that of the external air on the See also:outer surface of the membrana tympani, and that thus the membrana tympani will be rendered See also:independent of See also:variations of atmospheric pressure such as occur when we descend in a diving See also:bell or ascend in a See also:balloon . By a forcible expiration, the oral and nasal cavities being closed, air may be driven into the tympanum, while a forcible See also:inspiration (Valsalva's experiment) will draw air from that cavity . In the first See also:case, the membrana tympani will bulge outwards, in the second case inwards, and in both, from excessive stretching of the membrane, there will be partial deafness, especially for sounds of high See also:pitch . Permanent occlusion of the tube is one of the most See also:common causes of deafness . The membrana tympani is capable of being set into vibration by a sound of any pitch included in the range of perceptible sounds . It responds exactly as to number of vibrations (pitch), intensity of vibrations (intensity), and complexity of vibration (quality or timbre) . Consequently we can hear a sound of any given pitch, of a certain intensity, and in its own specific timbre or quality . Generally speaking, very high tones are heard more easily than See also:low tones of the same intensity . As the membrana tympani is not only fixed by its margin to a See also:ring or tube of See also:bone, but is also adherent to the handle of the malleus, which follows its movements, its vibrations meet with considerable resistance . This diminishes the intensity of its vibrations, and prevents also the • continued vibration of the membrane after an external pressure has ceased, so that a sound is not heard much longer than its See also:physical cause lasts . The tension of the membrane may be affected (r) by See also:differences of pressure on the two surfaces of the membrana tympani, as may occur during forcible expiration or inspiration, and (2) by See also:muscular action, due to See also:con-See also:traction of the tensor tympani muscle . This small muscle arises from the See also:apex of the petrous temporal and the See also:cartilage of the Eustachian tube, enters the tympanum at its anterior See also:wall, and is inserted into the malleus near its See also:root . The handle of the malleus is inserted between the layers of the membrana tympani, and, as the malleus and incus move round an See also:axis passing through the See also:neck of the malleus from before backwards, the action. of the muscle is to pull the membrana tympani inwards towards the tympanic cavity in the See also:form of a See also:cone, the meridians of which are not straight but curved, with convexity outwards . When the muscle contracts, the handle of the malleus is See also:drawn still farther inwards, and thus a greater tension of the tympanic membrane is produced . On relaxation of the muscle, the membrane returns to its position of equilibrium by its See also:elasticity and by the elasticity of the See also:chain of bones . This See also:power of varying the tension of the membrane is an accommodating mechanism for receiving and transmitting sounds of different pitch . With different degrees of tension it will See also:respond more readily to sounds of different pitch . Thus, when the membrane is tense, it will readily respond to high sounds, while relaxation will be the See also:condition most adapted for low tones . In addition, increased tension of the membrane, by increasing the resistance, will diminish the intensity of vibrations . This is especially the case for sounds of low pitch . The vibrations of the membrana tympani are transmitted tothe See also:internal ear partly by the air which the middle ear or tympanum contains, and partly by the chain of bones, consisting of the malleus, incus and stapes . Of these, transmission by the chain of bones is by far the most important . In birds and in the See also:amphibia, this chain is represented by a single See also:rod-like ossicle, the See also:columella, but in See also:man the two membranes—the membrana tympani and the membrane filling the fenestra ovalis—are connected by a See also:compound See also:lever consisting of three bones, namely, the malleus, or See also:hammer, inserted into the membrana tympani, the incus, or See also:anvil, and the stapes, or See also:stirrup, the See also:base of which is attached to a membrane covering the oval window . It must also be noted that in the transmission of vibrations of the membrana tympani to the fluid in the See also:labyrinth or internal ear, through the oval window, the chain of ossicles vibrates as a whole and acts efficiently, although its length may be only a fraction of the See also:wave-length of the sound transmitted . The chain is a lever in which the handle of the malleus forms the See also:long See also:arm, the fulcrum is where the See also:short See also:process of the incus abuts against the wall of the tympanum, while the long process of the incus, carrying the stapes, forms the short arm . The mechanism is a lever of the second See also:order . Measurements show that the ratio of the lengths of the two arms is as 1.5:1; the ratio of the resulting force at the stapes is therefore as 1:1.5; while the amplitudes of the movements at the tip of the handle of the malleus and the stapes is as 1.5:1 . Hence, while there is a diminution in See also:amplitude there is a gain in power, and thus the pressures are conveyed with See also:great efficiency from the membrana tympani to the labyrinth, while the amplitude of the oscillation is diminished so as to be adapted to the small capacity of the labyrinth . As the drum-head is nearly twenty times greater in See also:area than the membrane covering the oval window, with which the base of the stapes is connected, the See also:energy of the movements of the membrana tympani is concentrated on an area twenty times smaller; hence the pressure is increased thirtyfold (1.5)(20) when it acts at the base of the stapes . Experiments on the human ear have shown that the See also:movement of greatest amplitude was at the tip of the handle of the malleus, o•76 mm.; the movement of the tip of the long arm process of the incus was 0.2r mm.; while the greatest amplitude at the base of the stapes was only •0714 mm . Other observations have shown the movements at the stapes to have a still smaller amplitude, varying from o•oor to 0.032 mm . With tones of feeble intensity the movements must be almost infinitesimal . There may also be very See also:minute transverse movements at the base of the stapes . 3 . Transmission in the Internal Ear.—The internal ear is composed of the labyrinth, formed of the See also:vestibule or central See also:part, the semicircular canals, and the cochlea, each of which consists of an osseous and a membranous portion . The osseous labyrinth may be regarded as an osseous See also:mould in the petrous portion of the temporal bone, lined by tesselated endothelium, and containing a small quantity of fluid called the perilymph . In this mould, partially surrounded by, and to some extent floating in, this fluid, there is the membranous labyrinth, in certain parts of which we find the terminal apparatus in connexion with the auditory nerve, immersed in another fluid called the endolymph . The membranous labyrinth consists of a vestibular portion formed by two small See also:sac-like dilatations, called the saccule and the utricle, the latter of which communicates with the semicircular canals by five openings . Each canal consists of a tube, bulging out at each extremity so as to form the so-called See also:ampulla, in which, on a projecting See also:ridge, called the crista acustica, there are cells bearing long auditory hairs, which are the peripheral end-See also:organs of the vestibular branches of the auditory nerve . The cochlear See also:division of the membranous labyrinth consists of the ductus cochlearis, a tube of triangular form fitting in between the two cavities in the cochlea, called the scala vestibuli, because it commences in the vestibule, and the scala tympani, because it ends in the tympanum, at the round window . These two scalae communicate at the apex of the cochlea . The roof of the ductus cochlearis is formed by a thin membrane called the membrane of Reissner, while its See also:floor consists of the basilar membrane, on which we find the remarkable organ of See also:Corti, which constitutes the terminal organ of the cochlear division of the auditory nerve . It is sufficient to state here that this organ consists essentially of an arrangement of See also:epithelial cells bearing hairs which are in communication with the terminal filaments of this portion of the auditory nerve, and that See also:groups of these hairs pass through holes in a closely investing membrane, membrana reticularis, which may act as a damping apparatus, so as quickly to stop their movements . The ductus cochlearis and the two scalae are filled with fluid . Sonorous vibrations may reach the. fluid in the labyrinth by three different ways—(r) by the osseous walls of the labyrinth, (2) by the air in the tympanum and the round window, and (3) by the base of the stapes inserted into the oval window . When the head is plunged into See also:water, or brought into See also:direct contact with any vibrating See also:body, vibrations must be transmitted directly . Vibrations of the air in the mouth and in the nasal passages are also communicated directly to the walls of the cranium, and thus pass to the labyrinth . In like manner, we may experience auditive sensations, such as blowing, rubbing and hissing sounds, due to muscular contraction or to the passage of See also:blood in vessels See also:close to the auditory organ . It is doubtful whether any vibrations are communicated to the fluid in the labyrinth by the round window . Vibrations which cause hearing are communicated by the chain of bones . When the base of the stirrup is pushed into the oval window, the pressure in the labyrinth increases, and, as the only See also:mobile part of the wall of the labyrinth is the membrane covering the round window, this membrane is forced outwards; when the base of the stirrup moves outwards a See also:reverse action takes place . Thus the fluid of the labyrinth receives a See also:series of pulses isochronous with the movements of the base of the stirrup, and these pulses affect the terminal apparatus in connexion with the auditory nerve . The sacs of the internal ear, known as the utricle and saccule, receive the impulses of the base of the stapes . They are organs connected with the See also:perception of sounds as sounds, without reference to pitch or quality . For the See also:analysis of See also:tone a cochlea is necessary .
Even in mammals all the parts of the ear may be destroyed or affected by disease, except these sacs, without causing See also:complete deafness
.
It has been suggested by See also: According to this view, when animals became air-breathers, a part of the ear (the papilla acustica basilaris) was gradually evolved for the perception of delicate vibrations of sound . (See EQUILIBRIUM.) It is by means of the cochlea that we discriminate pitch, hear beats, and are affected by quality of tone . Since the See also:size of.the membranous labyrinth is so small, measuring, in man, not more than 2 in. in length bye in. in See also:diameter at its widest part, and since it is a chamber consisting partly of conduits of very irregular form, it is impossible to state accurately the course of vibrations transmitted to it by impulses communicated from the base of the stirrup . In the cochlea vibrations must pass from the saccule along the scala vestibuli to the apex, thus affecting the membrane of Reissner, which forms its roof; then, passing through the opening at the apex (the helicotrema), they must descend by the scala tympani to the round window, and affect in their passage the membrana basilaris, on which the organ of Corti is situated . From the round window impulses must be reflected backwards, but how they affect the advancing impulses is not known . But the problem is even more complex when we take into See also:account the fact that impulses are transthitted simultaneously to the utricle and to the semicircularcanals communicating with it by five openings . The mode of action of these vibrations or impulses upon the See also:nervous terminations is still unknown; but to appreciate critically the See also:hypothesis which has been advanced to explain it, it is necessary, in the first place, to refer to some of the general characters of auditory sensation . 4 . General Characters of Auditory Sensations.—Certain conditions are necessary for excitation of the auditory nprve sufficient to produce a sensation . In the first place, the vibrations must have a certain amplitude and energy; if too feeble, no impression will be produced . Various physicists have attempted to measure the sensitiveness of the ear by estimating the amplitude of the molecular movements necessary to See also:call forth the feeblest audible sound . Thus A . Topler and L . Boltzmann, on data founded on experiments with organ pipes, state that the ear is affected by vibrations of molecules of the air not more in amplitude than •0004 mm. at the ear, or o•x of the wave-length of See also:green See also:light, and that the energy of such a vibration on the drum-head is not more than See also:wig billionth kilog., or yth of that produced upon an equal surface of the retina by a single See also:candle at the same distance (See also:Ann. d . Phys. u . Chem., Leipzig . 1870, Bd. cxli . S . 321) . See also:Lord See also:Rayleigh, by two other methods, arrived at the conclusion "that the streams of energy required to See also:influence the See also:eye and ear are of the same order of magnitude." He estimated the amplitude of the movement of the aerial particles, with a sound just audible, as less than the ten-millionth of a centimetre, and the energy emitted when the sound was first becoming audible, at 42.1 ergs per second . He also states that in considering the amplitude or condensation in progressive aerial waves, at a distance of 27.4 metres from a tuning-See also:fork, the maximum condensation was =6•oXro 9 cm., a result showing "that the ear is able to recognize the addition or subtraction of densities far less than those to be found in our highest vacua " (Prot . See also:Roy . See also:Soc., 1877, vol. See also:xxvi. p . 248; See also:Land .
Edin. and Dub
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Phil
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Mag., 1894, vol. xxxviii. p
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366)
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In the next place, vibrations must have a certain duration to be perceived; and lastly, to excite a sensation of a continuous musical sound, a certain number of impulses must occur in a given See also:interval of See also:time
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The See also:lower limit is about 30, and the upper about 30,000 vibrations per second
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Below 30, the individual impulses may be observed, and above 30,000 few ears can detect any sound at all
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The extreme upper limit is not more than 35,000 vibrations per second
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Auditory sensations are of two kinds—noises and musical sounds
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Noises are caused by impulses which are not See also:regular in intensity or duration, or are not periodic, or they may be caused by a series of musical sounds occurring instantaneously so as to produce discords, as when we place our See also:hand at See also:random on the See also: Intensity depends on the amplitude of the vibration, and a greater or lesser amplitude of the vibration will cause a corresponding movement of the transmitting apparatus, and a corresponding intensity of excitation of the terminal apparatus . ' Pitch, as a sensation, depends on the length of time in which a single vibration is executed, or, in other words, the number of vibrations in a given interval of time . The ear is capable of appreciating the relative pitch or height of a sound as compared with another, although it may not ascertain precisely the See also:absolute pitch of a sound . What we call an acute or high tone is produced by a large number of vibrations, while a See also:grave or low tone is' caused by few . The musical tones which can be used with See also:advantage range between 40 and 4000 vibrations per second, extending thus from 6 to 7 octaves . According to E . H . See also:Weber, practised musicians can perceive a difference of pitch amounting to only the g1Tth of a semitone, but this is far beyond See also:average attainment . In a few individuals, and especially in See also:early See also:life, there may be an appreciation of absolute pitch . Quality or timbre (or Kiang) is that peculiar characteristic of a musical sound by which we may identify it as proceeding from a particular See also:instrument or from a particular human See also:voice . It depends on the fact that many waves of sound that reach the ear are compound wave systems, built up of constituent waves, each of which is capable of exciting a sensation of a See also:simple tone if it be singled out and reinforced by a resonator (see SOUND), and which may sometimes be heard without a resonator, after special practice and tuition . Thus it appears that the ear must have some arrangement by which it resolves every wave See also:system, however complex, into simple pendular vibrations . When we listen to a sound of any quality we recognize that it is of a certain pitch . This depends on the number of vibrations of one tone, predominant in intensity over the others, called the fundamental or ground tone, or first partial tone . The quality, or timbre, depends on the number and intensity of other tones added to it . These are termed See also:harmonic or partial tones, and they are related to the first partial or fundamental tone in a very simple manner, being multiples of the fundamental tone: thus Funda- Upper Partials or Harmonics. See also:mental Tone . Notes . . . do' do' See also:sole do3 mi3 sot' si53 do' re" mi4 Partial tones 1 2 3 4 5 6 7 8 9 10 Number of vibrations ) 33 66 99 132 165 198 231 264 297 330 When a simple tone, or one free from partials, is heard, it gives rise to a simple, soft, somewhat insipid sensation, as may be obtained by blowing across the mouth of an open See also:bottle or by a tuning-fork . The lower partials added to the fundamental tone give softness combined with richness; while the higher, especially if they be very high, produce a brilliant and thrilling effect, as is caused by the See also:brass See also:instruments of an See also:orchestra . Such being the facts, how may they be explained physiologically ? Little is yet known regarding the mode of action of the °ibrations of the fluid in the labyrinth upon the terminal apparatus connected with the auditory nerve . There can be no doubt that it is a See also:mechanical action, a communication of impulses to delicate See also:hair-like processes, by the movements of which the nervous filaments are irritated . In the human ear it has been estimated that there are about 3000 small See also:arches formed by the rods of Corti . Each See also:arch rests on the basilar membrane, and supports rows of cells having minute hair-like processes . It would appear also that the filaments of the auditory nerve terminate in the basilar membrane, and possibly they may be connected with the hair-cells . At one time it was supposed by See also:Helmholtz that these See also:fibres of Corti were elastic and that they were tuned for particular sounds, so as to form a regular series corresponding to all the tones audible to the human ear . Thus 2800 fibres distributed over the tones of seven octaves would give 400 fibres for each See also:octave, or nearly 33 for a semitone . Helmholtz put forward the hypothesis that, when a pendular vibration reaches the ear, it excites by sympathetic vibration the fibre of Corti which is tuned for its proper number of vibrations . If, then, different fibres are tuned to tones of different pitch, it is evident that we have here a mechanism which, by exciting different nerve fibres, will give rise to sensations of pitch . When the vibration is not simple but compound, in consequence of the blending of vibrations corresponding to various harmonics or partial tones, the ear has the power of resolving this compound vibration into its elements . It can only do so by different fibres responding to the constituent vibrations of the sound—one for the fundamental tone being stronger, and giving the sensation of a particular pitch to the sound, and the others, corresponding to the upper partial tones, being weaker, and causing undefined sensations, which are so blended together in consciousness as to terminate in a complex sensation of a tone of a certain quality or timbre . It would appear at first sight that 33 fibres of Corti for a semitone are not sufficient to enable us to detect all the gradations of pitch in that interval, sincel as has been stated above, trained musicians may distinguish a difference of nth of a semitone . To meet this difficulty, Helmholtz stated that if a sound is produced, the pitch of which may be supposed to come between two adjacent fibres of Corti, both of these will be set into sympathetic vibration, but the one which comes nearest to the pitch of the sound will vibrate with greater intensity than125 the other, and that consequently the pitch of that sound would be thus appreciated . These theoretical views of Helmholtz have derived much support from experiments of V . Hensen, who observed that certain hairs on the antennae of Mysis, a Crustacean, when seen with a low microscopic power, vibrated with certain tones produced by a keyed See also:horn . It was seen that certain tones of the horn set some hairs into strong vibration, and other tones other hairs . Each hair responded also to several tones of the horn . Thus one hair responded strongly to d# and d'#, more weakly to g, and very weakly to G . It was probably tuned to some pitch between d" and d" # . (Studien caber das Gehororgan der Decapoden, Leipzig, 1863.) Histological researches have led to a modification of this hypothesis . It has been found that the rods or arches of Corti are stiff structures, not adapted for vibrating, but apparently constituting a support for the hair-cells . It is also known that there are no rods of Corti in the cochlea of birds, which are capable nevertheless of appreciating pitch . Hensen and Helmholtz suggested the view that not only may the segments of the membrana basilaris be stretched more in the radial than in the See also:longitudinal direction, but different segments may be stretched radially with different degrees of tension so as to resemble a series of tense strings of gradually increasing length . Each See also:string would then respond to a vibration of a particular pitch communicated to it by the hair-cells . The exact mechanism of the hair-cells and of the membrana reticularis, which looks like a damping apparatus, is unknown . 5 . Physiological Characters of Auditory Sensation.—Under See also:ordinary circumstances auditory sensations are referred to the outer See also:world . When we hear a sound, we See also:associate it with some external cause, and it appears to originate in a particular place or to come in a particular direction . This feeling of exteriority of sound seems to require transmissior through the membrana tympani . Sounds which are sent through the walls of the cranium, as when the head is immersed in, and the external auditory canals are filled with, water, appear to originate in the body itself . An auditory sensation lasts a short time after the cessation of the exciting cause, so that a number of separate vibrations, each capable of exciting a distinct sensation if heard alone, may succeed each other so rapidly that they are fused into a single sensation . If we listen to the puffs of a syren, or to vibrating See also:tongues of low pitch, the single sensation is usually produced by about 30 or 35 vibrations per second; but when we listen to beats of considerable intensity, produced by two adjacent tones of sufficiently high pitch, the ear may follow as many as 132 intermissions per second . The sensibility of the ear for sounds of different pitch is not the same . It is more sensitive for acute than for grave sounds, and it is probable that the maximum degree of acuteness is for sounds produced by about 3000 vibrations per second, that is near fa tt . Sensibility as to pitch varies much with the individual . Thus some musicians may detect a difference of 1 olwlyth of the See also:total number of vibrations, while other persons may have difficulty in appreciating a semitone . 6 . See also:Analytical Power of the Ear.—When we listen to a compound tone, we have the power of picking out these partials from the general See also:mass of sound . It is known that the frequencies of the partials as compared with that of the fundamental tone are simple multiples of the frequency of the fundamental, and also that physic-ally the waves of the partials so blend with each other as to produce waves of very complicated forms . Yet the ear, or the ear anth he See also:brain together, can resolve this complicated wave-form into its constituents, and this is done more easily if we listen to the sound with resonators, the pitch of which corresponds, or nearly corresponds, to the frequencies of the partials . 
Much discussion has taken place as to how the ear accomplishes this analysis
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All are agreed that there is a complicated apparatus in the cochlea which may serve this purpose; but while some are of See also:opinion that this structure is sufficient, others hold that the analysis takes place in the brain
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When a complicated wave falls on the drum-head, it must move out and in in a way corresponding to the variations of pressure, and these variations will, in a single vibration, depend on the greater or less degree of complexity of the wave
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Thus a single tone will cause a movement like that of a pendulum, a simple pendular vibration,
while a complex tone, although occurring in the same duration of time, will cause the drum-head to move out and in in a much more complicated manner
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The complex movement will be conveyed to the base of the stapes, thence to the vestibule, and thence to the cochlea, in which we find the ductus cochlearis containing the organ of Corti
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It is to be noted also that the parts in the cochlea are so small as to constitute only a fraction of the wave-length of most tones audible to the human ear
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Now it is evident that the cochlea must act either as a whole, all the nerve fibres being affected by any variations of pressure, or the nerve fibres may have a selective action, each fibre being excited by a wave of a definite See also:period, or there may exist small vibratile bodies between the nerve filaments and the pressures sent into the organ
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The last hypothesis gives the most rational explanation of the phenomena, and on it is founded a theory generally accepted and associated with the names of See also: (5) If a simple tone falls on the ear, there is a pendular movement of the base of the stapes, which will affect all the parts, causing them to move; but any part whose natural period is nearly the same as that of the sound will respond on the principle of sympathetic resonance, a particular nerve filament or nerve filaments will be affected, and a sensation of a tone of definite pitch will be experienced, thus accounting for discrimination in pitch . (6) Intensity or loudness will depend on the amplitude of movement of the vibrating body, and consequently on the intensity of nerve stimulation . (7) If a compound wave of pressure be communicated by the base of the stapes, it will be resolved into its constituents by the vibrators corresponding to tones existing in it, each picking out its appropriate portion of the wave, and thus irritating corresponding nerve filaments, so that nervous impulses are transmitted to the brain, where they are fused in such a way as to give rise to a sensation of a particular quality or See also:character, but still so imperfectly fused that each constituent, by a strong effort of See also:attention, may be specially recognized " (article " Ear," by M'Kendrick, Schafer's See also:Text-See also:Book, loc. cit.) . The structure of the ductus cochlearis meets the demands of this theory, it is highly differentiated, and it can be shown that in it there are a sufficient number of elements to account for the delicate appreciation of pitch possessed by the human ear, and on the basis that the highly trained ear of a violinist can detect a difference of Q,th of a semitone (M'Kendrick, Trans . Roy . Soc . Ed., 1896, vol. xxxviii. p . 78o; also Schafer's Text-Book, loc. cit.) . Measurements of the cochlea have also shown such differentiation as to make it difficult to imagine that it can act as a whole . A much less complex organ might have served this purpose (M'Kendrick, op. cit.) . The following table, given by Retzius (Das Gehororgan der Wirbelthiere, Bd. ii . S . 356), shows differentiations in the cochlea of man, the See also:cat and the See also:rabbit, all of which no doubt hear tones, although in all See also:probability they have very different See also:powers of discrimination: Man . Cat . Rabbit . Ear-See also:teeth . 2,490 2,430 1,550 Holes in habenula for nerves 3,985 2,780 1,650 Inner rods of Corti's organ 5,590 4,700 2,800 Outer rods of Corti's organ 3 48 3,300 1,900 Inner hair-cells (one See also:row) 3,487 2,600 1,600 Outer hair-cells (several rows) 11,750 9,900 6,100 Fibres in basilar membrane . 23,750 15,700 10,500 7 . Dissonance.—The theory can also be used to explain dissonance . When two tones sufficiently near in pitch are simultaneously sounded, beats are produced . If the beats are few in number they can be counted, because they give rise to separate and distinct sensations; but if they are numerous they blend so as to give roughness or dissonance to the interval . The roughness or dissonance is most disagreeable with about 33 beats falling on the ear per second . When two compound tones are sounded, say a See also:minor third on a See also:harmonium in the lower part of the See also:keyboard, then we have beats not only between the primaries, but also between the upper partials of each of the primaries . The beating distance may, for tones of See also:medium pitch, be fixed at about a minor third, but this interval will expand for intervals on low tones and See also:contract for intervals on high ones . This explains why the same interval in the lower part of the See also:scale may give slow beats that are not disagreeable, while in the higher part it may cause harsh and unpleasant dissonance . The partials up to the seventh are beyond beating distance, but above this theycome close together . Consequently instruments (such as tongues, or reeds) that abound in upper partials cause an intolerable dissonance if one of the primaries is slightly out of tune . Some intervals are pleasant and satisfying when produced on instruments having few partials in their tones . These are concords . Others are less so, and they may give rise to an uncomfortable sensation . These are discords . In this way unison, i, minor third s, See also:major third 6 See also:fourth g, fifth 2, minor See also:sixth t, major sixth and octave -, are all concords; while a second , minor seventh 1b6 and major seventh Y, are discords . Helmholtz compares the sensation of dissonance to that of a flickering light on the eye . " Something similar I have found to be produced by simultaneously stimulating the skin, or margin of the lips, by bristles attached to tuning-forks giving forth beats . If the frequency of the forks is great, the sensation is that of a most disagreeable tickling . It may be that the instinctive effort at analysis of tones close in pitch causes the disagreeable sensation " (Schafer's Text-Book, op. cit. p . 1187) . 8 . Other Theories.—In 1865 See also:Rennie objected to the analysis theory, and urged that the cochlea acted as a whole (Ztschr. f. See also:rat . Med., Dritte Reihe, Bd. See also:xxiv . Heft 1, S . 12-64) . This view was revived by Voltolini (See also:Virchow's Archiv, Bd. c . S . 27) some years later, and in 1886 it was urged by E . See also:Rutherford (See also:Rep . Brit . Assoc . Ad . Sc., 1886), who compared the action of the cochlea to that of a See also:telephone See also:plate . According to this theory, all the hairs of the auditory cells vibrate to every See also:note, and the hair-cells transform sound vibrations into nerve vibrations or impulses, similar in frequency, amplitude and character to the sound vibrations . There is no analysis in the peripheral organ . A . D . See also:Waller, in 1891 (Prot . Physiol . Soc., See also:Jan . 2o, 1891) suggested that the basilar membrane as a whole vibrates to every note, thus repeating the vibrations of the membrana tympani ; and since the hair-cells move with the basilar membrane, they produce what may be called pressure patterns against the tectorial membranes, and filaments of the auditory nerve are stimulated by these pressures . Waller admits a certain' degree of peripheral analysis, but he relegates ultimate analysis to the brain . These theories, dispensing with peripheral analysis, leave out of account the highly complex structure of the cochlea, or, in other words, they assign to that structure a comparatively simple function which could be performed by a simple membrane capable of vibrating . We find that the cochlea becomes more elaborate as we ascend the scale of animals, until in man, who possesses greater powers of analysis than any other being, the number of hair-cells, fibres of the basilar membrane and arches of Corti are all much increased in number (see Retzius's table, supra) . The principle of sympathetic resonance appears, therefore, to offer the most likely See also:solution of the problem . See also:Hurst's view is that with each movement of the stapes a wave is generated which travels up the scala vestibuli, through the helicotrema into the scala tympani and down the latter to the fenestra rotunda . The wave, however, is not merely a movement of the basilar membrane, but an actual movement of fluid or a transmission of pressure . As the one wave ascends while the other descends, a pressure of the basilar membrane occurs at the point where they meet ; this causes the basilar membrane to move to-wards the tectorial membrane, forcing this membrane suddenly against the apices of the hair-cells, thus irritating the nerves . The point at which the waves meet will depend on the time interval between the waves (Hurst, " A New Theory of Hearing," Trans . Biol . Soc . See also:Liverpool, 1895, vol. ix. p . 321) . More recently Max See also:Mayer has advanced a theory somewhat similar . He supposes that with each movement of the stapes corresponding to a vibration, a wave travels up the scala vestibuli, pressing the basilar membrane down-wards . As it meets with resistance in passing upwards, its amplitude therefore diminishes, and in this way the distance up the scala through which the wave progresses will be determined by its amplitude . The wave in its progress irritates a certain number of nerve terminations, consequently feeble tones will irritate only those nerve fibres that are near the fenestra ovalis, while stronger tones will pass farther up and irritate a larger number of nerve fibres the same number of times per unit of time . Pitch, according to this view, depends on the number of stimuli per second, while loudness depends on the number of nerve fibres irritated . Mayer also applies the theory to the explanation of,the powers of the cochlea as an analyser, by supposing that with a compound tone these are at See also:maxima and minima of stimulation . As the compound wave travels up the scala, portions of the wave corresponding to maxima and minima See also:die away in consecutive series, until only a maximum and minimum are See also:left; and, finally, as the wave travels farther, these also disappear . Wilts each maximum and minimum different parts of the basilar membrane are affected, and affected a different number of times per second, according to the frequencies of the partials existing in the compound tone . Thus with a fifth, 2 : 3, there are three maxima and three minima; but the compound tone is resolved into three tones having vibration frequencies-in the ratio of 3 : 2 : 1 . According to Mayer, we actually hear when a fifth is sounded tones of the relationship of 3 : 2 : 1, the last (1) being the See also:differential tone . He holds, also, that combinational tones are entirely subjective (Max Mayer, Ztschr. f . Psych. and Phys. d . Sinnesorgane, Leipzig, Bd. xvi. and xvii . ; also Verhandl. d. physiolog . Gesellsch. zu See also:Berlin, Feb . 18, 1898, S . 49)_ Two fatal objections can be urged to these theories, namely, first, it is impossible to conceive of minute waves following each other in rapid See also:succession in the minute tubes forming the scalae-the length of the scala being only a very small part of the wave-length of the sound; and, secondly, neither theory takes into account the differentiation of structure found in the epithelium of the organ of Corti . Each push in and out of the base of the stapes must cause a movement of the fluid, or a pressure, in. the scalae as a whole . There are difficulties in the way of applying the resonance theory to the perception of noises . Noises have pitch, and also each See also:noise has a special character; if so, if the noise is analysed into its constituents, why is it that it seems impossible to analyse a noise, or to perceive any musical See also:element in it ? Helmholtz assumed that a sound is noisy when the wave is irregular in See also:rhythm, and he suggested that the crista and macula acustica, structures that exist not in the cochlea but in the vestibule, have to do with the perception of noise . These structures, however, are concerned rather in the sense of the perception of equilibrium than of sound (see EQUILIBRIUM) . 9 . Hitherto we have considered only the audition of a single sound, but it is possible also to have simultaneous auditive sensations, as in musical See also:harmony . It is difficult to ascertain what is the limit beyond which distinct auditory sensations may be perceived . We have in listening to an orchestra a multiplicity of sensations which produces a total effect, while, at the same time, we can with ease single out and See also:notice attentively the tones of one or two special instruments . Thus the See also:pleasure of See also:music may arise partly from listening to simultaneous, and partly from the effect of contrast or See also:suggestion in passing through successive, auditory sensations . The principles of harmony belong to the subject of music (see HARMONY), but it is necessary here briefly to refer to these from the physiological point of view . If two musical sounds reach the ear at the same moment, an agreeable or disagreeable sensation is experienced, which may be termed a See also:concord or a discord, and it can be shown by experiment with the syren that this depends upon the vibrational See also:numbers of the two tones . The octave (I : 2), the twelfth (I : 3) and See also:double octave (i : 4) are absolutely consonant sounds; the fifth (2 :3) is said to be perfectly consonant; then follow, in the direction of dissonance, the fourth (3 : 4), major sixth (3: 5), major third (4:5), minor sixth (5 : 8) and the minor third (5 : 6) . Helmholtz has attempted to account for this by the application of his theory of beats . Beats are observed when two sounds of nearly the same pitch are produced together, and the number of beats per second is equal to the difference of the number of vibrations of the two sounds . Beats give rise to a peculiarly disagreeable intermittent sensation . The maximum roughness of beats is attained by 33 per second; beyond 132 per second, the individual impulses are blended into one See also:uniform auditory sensation . When two notes are sounded, say on a piano, not only may the first, fundamental or See also:prime tones See also:beat, but partial tones of each of the primaries may beat also, and as the difference of pitch of two simultaneous sounds augments, the number of beats, both of prime tones and of harmonics, augments also . The physiological effect of beats, though these may not be individually distinguishable, is to give roughness to the ear . If harmonics or partial tones of prime tones coincide, there are no beats; if they do not coincide, the beats produced will give a character of roughness to the interval . Thus in the octave and twelfth, all the partial tones of the acute sound coincide with the partial tones of the grave sound; in the fourth, major sixth and major third, only two pairs of the partial tones coincide, while in the minor sixth, minor third and minor seventh only one pair of the harmonics coincide It is possible by means of beats to measure the sensitiveness of the ear by determining the smallest difference in pitch that may give rise to a See also:heat . In no part of the scale can a difference smaller than 0.2 vibration per second be distinguished . The sensitiveness varies with pitch . Thu |